An impact absorbing structure and a helmet comprising such a structure

ABSTRACT

An impact absorbing structure comprises a unitary material formed as a stretch-dominated hollow cell structure and a helmet comprising such a structure as an inner impact resistant liner.

FIELD

The present invention relates to an impact absorbing structure. Moreparticularly, the present invention relates to a hollow-cell impactabsorbing structure. Even more particularly, the present inventionrelates to an impact absorbing structure formed as a stretch-dominatedhollow-cell structure. The present invention also relates to impactabsorbing structures where the impact surface is curved, such as asports helmet or aerospace nose bumpers, at least part of the structureformed from a hollow-cell impact absorbing structure, and even moreparticularly a stretch-dominated hollow-cell impact absorbing structure.

BACKGROUND

Injury to a person or damage to an object can occur when the person orobject is subjected to an impact of sufficient force. Considerabledevelopmental effort has been expended in order to produce materials andstructures that provide protection from potentially damaging orinjurious impacts.

Impact protection is particularly important for preventing head injury.A blow to the head can result in severe traumatic brain injury (TBI). Itis common for brain trauma to occur as a consequence of either a focalimpact upon the head, or by a sudden acceleration/deceleration withinthe cranium, or from a combination of both impact and movement.Traumatic brain injury can cause long-term issues, and there are limitedtreatment options.

One of the most common causes of head injury is participation in sports.For example, a fall from a bicycle when riding may result in the headstriking against a solid unyielding object or surface such as a roadsurface or similar. In order to help prevent injury, helmet usage iscustomary or mandatory in many sports such as bicycle, motorcycle andhorse riding, rock climbing, American football and also winter or icesports such as skating, ice hockey, and skiing. Another common cause ofhead injury is an impact caused by a falling object on a building orconstruction site.

Sports helmets and safety helmets are individually designed so as to beparticularly suited to their particular use. However, most or all of thehelmets have common design elements such as a hard outer shell (formedfrom a stiff thermoplastic or composite) and a lining/liner, softer thanthe outer shell, but still stiff enough to retain it's shape whenunsupported. In combination, the shell and liner act to absorb the forceof an impact and to help prevent this force being transmitted to thehead and brain. Virtually all helmets use expanded polystyrene as theenergy absorbing liner. The expanded polystyrene is formed as a unitarystructure (that is, without gaps) in the required shape.

U.S. Pat. No. 3,447,163 describes and shows a safety or crash helmetintended for use by motorcyclists and/or racing motorists. The helmethas an outer shell formed as a double-skinned member, the two skins ofthe shell joined to one another around the periphery of the shell by agently curved peripheral portion that has no sharp edges, and the spacebetween the skins contains a layer of a honeycomb type of material, thecells of the honeycomb layer filled with an energy-absorbing foamedmaterial.

U.S. Pat. No. 7,089,602 describes and shows an impact absorbing, modularhelmet having layers on the outer side of a hard casing that increasethe time of impact with the intention of reducing the intensity of theimpact forces. The layers are made up of a uniformly consistent impactabsorbing polymer material, a polymer layer filled with air or a polymerstructure. These impact-absorbing layers can also be made and used as anindependent, detachable, external protective cover that can be attachedover a hard casing helmet.

U.S. Pat. No. 6,247,186 describes and shows a helmet having a housing,an inner impact resistant layer shaped to the head of rider, aprotective covering spaced above and formed integrally with the housing,and a chamber enclosed by the housing and protective covering that isopen in the front for ventilation. The chamber has a net strap in thefront side for preventing foreign objects from entering and one or moreinner channels in communication with the inner space of helmet through apassageway. In use, fresh air flows through the passageway and into theimpact resistant layer.

Sports helmets and safety helmets often have to be worn for extendedperiods, and the weight of the helmet is an important designconsideration. When designing a helmet, there will generally be atrade-off between the overall weight (and shape and size) of the helmet,and the impact-absorbing properties. Increasing the amount ofimpact-absorbing material will increase the overall weight of thehelmet, and may also result in an increase in the external dimensions,which can in turn make wearing the helmet relatively more unwieldy anduncomfortable to wear, especially where aerodynamic considerations mayalso be important. Conversely, impact protection can be compromised ifthe helmet has too little impact-absorbing material.

Foams such as the foams used in helmets are typically excellent energyabsorbers because they are characterised by a long plateau stress, andin most impacts the area is constant so the stress can be directlyconverted to force, providing a long plateau force. This means all theenergy can be absorbed whilst maintaining a low peak force andacceleration, optimal in reducing brain damage. However, in an ovalshape helmet, the area when crushing is not constant.

In this specification where reference has been made to patentspecifications, other external documents, or other sources ofinformation, this is generally for the purpose of providing a contextfor discussing the features of the invention. Unless specifically statedotherwise, reference to such external documents is not to be construedas an admission that such documents, or such sources of information, inany jurisdiction, are prior art, or form part of the common generalknowledge in the art.

SUMMARY

It is an object of the present invention to provide a range of optimisedimpact absorbing structures for improved impact absorption, or which atleast provide the public or industry with a useful choice. It is afurther object of the invention to provide a range of optimised impactabsorbing structures that can be used to reduce traumatic brain injuryby reducing peak acceleration and force to the brain and directingenergy away from vulnerable areas of the brain, or which at leastprovide the public or industry with a useful choice. It is a yet stillfurther object of the invention to provide a helmet at least partlyformed from an optimised impact absorbing structure that assists withreducing traumatic brain injury by reducing peak acceleration and forceto the brain and directing energy away from vulnerable areas of thebrain, or which at least provides the public or industry with a usefulchoice. It is a yet still further object of the present invention toprovide a method of optimising an impact absorbing structure forimproved impact absorption.

The term “comprising” as used in this specification and indicativeindependent claims means “consisting at least in part of”, and isintended as an inclusive rather than exclusive term. When interpretingeach statement in this specification and indicative independent claimsthat includes the term “comprising”, features other than that or thoseprefaced by the term may also be present. Related terms such as“comprise” and “comprises” are to be interpreted in the same manner.

As used herein the term “and/or” means “and” or “or”, or both.

As used herein “(s)” following a noun means the plural and/or singularforms of the noun.

Accordingly, in a first aspect the present invention may broadly be saidto consist in an impact absorbing structure, comprising a unitarymaterial formed as a stretch-dominated hollow cell structure.

In an embodiment, substantially all the cells of the hollow cellstructure are 2D hollow-cells.

In an embodiment, substantially all the cells are aligned substantiallyout of plane.

In an embodiment, the cells are formed as a micro-truss lattice.

In an embodiment, the cells are formed as a crystal lattice structure.

In an embodiment, at least a plurality of the cells are configured totessellate.

In an embodiment, at least a plurality of the cells are configured totessellate with a cell axis normal to the surface or out-of-plane.

In an embodiment, at least a plurality of the cells are hexagonal.

In an embodiment, at least a plurality of the cells are triangular.

In an embodiment, at least a plurality of the cells are square.

In an embodiment, at least a plurality of the cells are a combination ofoctagons and squares co-located in a tessellating pattern.

In an embodiment, the unitary material is formed to have a relativedensity substantially between 0.05 and 0.15.

In an embodiment, the cell shape, size, cell wall thickness, cell widthand cell length can be freely varied relative to one another.

In an embodiment, the ratio of cell wall thickness to cell length issignificantly small.

In an embodiment, the wall has a maximum thickness of substantially 1mm.

In an embodiment, the unitary material is a polymer material.

In an embodiment, the unitary material is an elastomer.

In an embodiment, the unitary material is elastic-plastic andelastic-brittle.

In an embodiment, the unitary material is Nylon 11.

In an embodiment, the unitary material is ST Elastomer.

In an embodiment, the hollow cell structure is manufactured by LaserSintering.

In a second aspect, the present invention may broadly be said to consistin a helmet, comprising an inner impact resistant liner at least partlyformed form an impact absorbing structure as claimed in any one of thepreceding statements.

In an embodiment, the helmet further comprises an outer shell formed tosubstantially cover the inner impact resistant liner.

In an embodiment, the outer shell is at least partly formed from acomposite material.

In an embodiment, the outer shell is at least partly formed from athermoplastic material.

In an embodiment, at least one vent slot is formed in the outer shell.

In a third aspect, the invention may broadly be said to consist in amethod of optimising an impact absorbing structure for improved impactabsorption, comprising the steps of:

-   -   (i) choosing a material;    -   (ii) forming the material into a stretch-dominated hollow cell        structure.

In an embodiment of the method, substantially all the cells of thehollow cell structure are formed as 2D hollow-cells.

In an embodiment of the method, substantially all the cells are formedso as to be aligned substantially out of plane.

In an embodiment of the method, the cells are formed as a micro-trusslattice.

In an embodiment of the method, the cells are formed as a crystallattice structure.

In an embodiment of the method, at least a plurality of the cells areformed so as to tessellate.

In an embodiment of the method, at least a plurality of the cells areformed so as to tessellate with a cell axis normal to the surface orout-of-plane.

In an embodiment of the method, the hollow cells are formed to have atopology that propagates radially to a curved surface.

In an embodiment of the method, at least a plurality of the cells areformed as hexagons.

In an embodiment of the method, at least a plurality of the cells areformed as triangles.

In an embodiment of the method, at least a plurality of the cells areformed as squares.

In an embodiment of the method, at least a plurality of the cells areformed as a combination of octagons and squares co-located in atessellating pattern.

In an embodiment of the method, the material is formed in such a mannerthat the material has a relative density substantially between 0.05 and0.15.

In an embodiment of the method, the cells are formed so that the cellshape, size, cell wall thickness, cell width and cell length can befreely varied relative to one another.

In an embodiment of the method, the cells are formed so that the ratioof cell wall thickness to cell length is significantly small.

In an embodiment of the method, the cells are formed so that the wallhas a maximum thickness of substantially 1 mm.

In an embodiment of the method, the unitary material is a polymermaterial.

In an embodiment of the method, the unitary material is an elastomer.

In an embodiment of the method, the unitary material is elastic-plasticand elastic-brittle.

In an embodiment of the method, the unitary material is Nylon 11.

In an embodiment of the method, the unitary material is ST Elastomer.

In an embodiment of the method, the hollow cell structure ismanufactured by Laser Sintering.

With respect to the above description then, it is to be realised thatthe optimum dimensional relationships for the parts of the invention, toinclude variations in size, materials, shape, form, function and mannerof operation, assembly and use, are deemed readily apparent and obviousto one skilled in the art, and all equivalent relationships to thoseillustrated in the drawings and described in the specification areintended to be encompassed by the present invention.

This invention may also be said broadly to consist in the parts,elements and features referred to or indicated in the specification ofthe application, individually or collectively, and any or allcombinations of any two or more said parts, elements or features, andwhere specific integers are mentioned herein which have knownequivalents in the art to which this invention relates, such knownequivalents are deemed to be incorporated herein as if individually setforth.

Therefore, the foregoing is considered as illustrative only of theprinciples of the invention. Further, since numerous modifications andchanges will readily occur to those skilled in the art, it is notdesired to limit the invention to the exact construction and operationshown and described, and accordingly, all suitable modifications andequivalents may be resorted to, falling within the scope of theinvention.

BRIEF DESCRIPTION OF THE FIGURES

Further aspects of the invention will become apparent from the followingdescription which is given by way of example only and with reference tothe accompanying drawings which show an embodiment of the device by wayof example, and in which:

FIGS. 1a-c show schematic views of single cells that form part of acellular solid, showing the joints j and struts s which the cell shareswith adjoining cells, the struts s forming surrounding faces thatenclose the cells, FIG. 1a showing a bending-dominated structure wherethe joints are locked and the frame bends as the structure is loaded,stretch-dominated structures shown in FIGS. 1b and 1c where the memberscarry tension or compression when loaded, giving higher modulus andstrength.

FIGS. 2a and 2b show plots summarising the difference between stretchand bending-dominated structures in terms of relative modulus E/E_(s)and relative strength σ/σ_(s) against relative density p/p_(s).

FIG. 3 shows a perspective view from above, looking downwards andsideways, of a honeycomb hollow cell structure according to anembodiment of the present invention.

FIG. 4 shows a top view from directly above of the hollow cell structureof FIG. 3.

FIG. 5 shows a section of a periodic lattice of hexagonal cells, showingthe positions of the joints j and struts s for this stretch dominatedstructure.

FIG. 6 shows a perspective view from one side of an inner impactresistant liner of a cycle helmet, the inner impact resistant linerformed from a hollow cell structure similar to that shown in FIGS. 3, 4,and 5, the liner shaped to follow and substantially conform to the topportion of a user's head.

FIG. 7 shows a perspective view directly from the rear of the innerimpact resistant liner of FIG. 3, with an outer shell covering the innerimpact resistant liner, vent slots formed in the outer shell to allowair to circulate within the inner impact resistant liner.

FIG. 8 shows a perspective schematic view from the front and to one sideof a test rig used to test samples of a hollow cell structure.

FIG. 9 is a graph showing the Head Injury Criterion (HIC) and peakacceleration for a range of test samples.

FIGS. 10 to 12 show test samples of honeycomb hollow cell structureaccording to embodiments of the present invention post-testing, eachsample having a different relative density, FIG. 10 showing brittlefailure at a relative density of 0.111, FIG. 11 showing plastic work ata relative density of 0.143, and FIG. 12 showing linear elasticdeformation at a relative density of 0.25.

FIG. 13 shows a graphical plot of energy per volume vs peak accelerationfor a range of test materials and conditions.

FIG. 14 shows graphical plots of acceleration vs time and force vsdisplacement for test pieces formed from Nylon 11, Elasto and EPS.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described withreference to the figures. As outlined above in the background sectioncertain structures are known to be suited for impact absorption.However, it has not been fully understood how these could be structuredin order to optimise the impact absorption properties. Outlined beloware examples of optimised structures for improved impact absorption.These can be used to form items intended to reduce traumatic braininjury such as bicycle helmets. A method of optimising the structure forimproved impact absorption is also described.

Previously, it has been assumed when assessing energy dissipation inhelmets or similar impact absorbing structures that the liner foam isentirely responsible for dissipating the impact energy. The reactionforce is determined by the compressive strength of the foam. A foamlattice is assumed to have a flat plateau compressive strength over itsdensification strain. However, the foam only provides an idealforce-displacement curve if the compressed region is uniform in area. Ina curved structure such as a helmet with a substantially ovoid shape,the impact area or crush area is not constant, or planar: the contactarea increases with displacement. This causes the reaction force to alsoincrease. Furthermore, if the curved helmet surface impacts anothercurved surface, the force-displacement gradient will be further reduced.The consequence of this is that a foam liner needs to be thicker inorder to provide adequate energy absorption by maintaining the peakacceleration below the safety legislation. The consistent plateau stressof foam limits it's effectiveness as an energy absorbing structure whenused as a curved structure (such as for example in a helmet) due to theinherent curved contact surface. The other assumption is that the lineris formed as a unitary structure (that is, without gaps).

As outlined below, it is possible to create a structure that has astiffness and strength higher than would otherwise be the case ifcreating the structure as a unitary structure formed from e.g. foam, fora given relative density p/p_(s) (where p is the density of the foam andp_(s) that of the bulk material), and this allows more energy to bedissipated per volume. It is also possible to create a structure thatprovides an initially high strength when the contact area is very lowand which has a gradual post-yield softening proportional to theincrease in contact area.

This is achieved by forming the impact-absorbing structure as ahollow-cell structure which is stretch-dominated, such as for example amicro-truss lattice or out-of-plane honeycomb.

In this type of structure, the mechanism of deformation involves ‘hard’modes such as compression and tension rather than bending. For the samerelative density as foam, stretch dominated structures have acomparatively higher modulus and yield stress. This is discussed below.

In stretch-dominated hollow-cell structures, yield stress occurs due tolocalised plastic buckling and brittle collapse of the struts. This isalso known as the bifurcation point because the structure becomesunstable and a post yield softening regime ensues.

The stress rises steeply at the densification strain (ε_(d)), which canbe calculated from

$ɛ_{d} = {1 - {\left( \frac{\rho}{\rho_{s}} \right)/\left( \frac{\rho_{crit}}{\rho_{s}} \right)}}$

where p is the density of the structure and p_(s) that of the bulkmaterial, and where p_(crit)/p_(s) is the relative density (or volumefraction solid) at which the structure locks up.

Apart from light weight and ventilation, there are potentially two keybenefits of using hollow-cell stretch-dominated structures as an impactabsorbing structure. Firstly, the post-yield softening counteracts thearea increase of a oval shaped helmet dissipating energy at a moreuniform plateau force. Secondly, for a given yield stress, the relativedensity of a stretch-dominated structure can be much lower providing agreater densification strain and therefore increasing the potentialenergy dissipated over the same displacement.

One particular form of stretch-dominated structure is a cellular solid.A cellular solid is one made up of an interconnected network of solidstruts or plates that form the edges and faces of cells. Typically themechanical behaviour of cellular solids can be distinguished bybending-(foam) and stretch-(lattice) dominated mechanisms. The Maxwellstability criterion is used to distinguish between bending- andstretch-dominated structures. Cellular solids can be thought of asjoints j, joined by struts s, which surround faces that enclose cells,as shown in FIG. 1.

The effect of material in the faces stiffens the structure by aconstant. In FIG. 1 a, when the frame is compressed it has no stiffnessor strength in the loading direction. If the joints are frozen (locked)the frame in FIG. 1a will bend as the structure is loaded and can becalled a bending-dominated structure. In the stretch-dominatedstructures in FIG. 1b and 1 c, the members carry tension or compressionwhen loaded, giving higher modulus and strength. This is shown in FIGS.2a and 2b , which summarise the difference between stretch andbending-dominated structures in terms of relative modulus E/E_(s) andrelative strength σ/σ_(s) against relative density p/p_(s). In thestructures in FIGS. 1a and 1 b, the structure carries self-stress, whichmeans the struts carry stress even though the structure carries noexternal load (this is prevalent in FIG. 1c ). For example if thevertical strut is shortened, it pulls the other struts into compression.

The two key benefits of using stretch-dominated structures as impactabsorbing structures are as follows: firstly, the post-yield softeningcounteracts the area increase of a oval shaped helmet dissipating energyat a more uniform plateau force, and; secondly, for a given yieldstress, the relative density of a stretch-dominated structure can bemuch lower providing a greater densification strain and thereforeincreasing the potential energy dissipated over the same displacement.This is discussed in detail in Appendix E.

In the embodiments described below, the impact absorbing structure isformed as a lattice—i.e. from interconnected hollow cells. Also, inorder to simplify manufacture, a periodic lattice is described (i.e. thecells are regularly shaped and sized). Hexagonal cells were used as thisshape has the largest number of side and which will still regularlytessellate—i.e. without requiring a second shape to fill gaps (forexample, if a regular octagon lattice was chosen, a regular square shapewould be inherent). Hexagonal honeycomb cells have the highest number ofcell walls for each cell, and therefore the lowest connectivity. whichhas been shown to be effective in high specific strength.

Other shapes (e.g. triangles and squares) will also tessellate, but havefewer sides. However, the number of sides has been shown to correlatepositively with the dissipated energy per unit mass of the structure(SAE).

The types of lattice structure described above can be generallydescribed as 2D hollow cell structures. Where these are referred to inthis specification, this indicates a three-dimensional structure, withthe cells of the structure formed in such a way as to have depth, but sothat when viewed at a certain angle the cells will have a uniform oridentical cross-section at any position perpendicular to the view angle.That is, a cross section taken at any position would be identical to onetaken at any other position. For example, a honeycomb cell structureviewed in plan or from directly above will provide a uniformcross-section at any depth through the cells. This can be translated tocurved shapes such as the ovoid shape necessary to form a helmet, forexample. When viewed at any particular point looking inwards towards thecentre of the interior, the cells will appear identical to those viewedfrom another point also looking inwards towards the centre of theinterior.

It should be noted that other types of structure, formed as stretchdominated structures, will also provide the same advantages. Forexample, 3-D stretch-dominated structures such as a truss structure or astructure similar to a crystal lattice structure can also be formed,which will provide the same impact absorption benefits.

As shown in FIGS. 3 and 4 the hollow cell stretch dominated structure 1used in a first embodiment of the present invention is a unitarymaterial formed into a honeycomb structure. It is preferred that thecells are hexagonal, as hexagonal cells 2 such as those used in thehollow cell structure 1 tessellate and so form a structure where eachcell wall is common with an adjacent cell. A grid formed from hexagonalcells also provides a balance between overall grid density (the totalamount of material), and the layout/location of the cell wall materialand the empty space which the cell walls encompass. That is,tessellation is achieved with the cell walls distributed over a givenplanar or curved surface as evenly as possible, with no overloaded focalareas, or over-large uncovered areas.

Hexagonal honeycomb can be thought of as a stretch dominated structureby applying the Maxwell criterion:

M _(honeycomb)=30−3×12+6=0

FIG. 4 shows a section of a periodic lattice of hexagonal cells, showingthe positions of the joints j and struts s for this stretch-dominatedstructure.

In practical use, and when experiencing an impact, the honeycombstructure will experience both in-plane and out-of-plane loading.Stretch-dominated structures such as the hexagonal hollow cell structure1 are generally used in a planar or sheet form, either flat or curved,and the impacts received by the hollow cell structure have a primaryforce component directed into the plane perpendicular to the point ofimpact. That is, in the opposite direction to out-of-plane arrow 3 inFIG. 1. However, as noted there will frequently be a force component atan angle to this, and the theory behind this is discussed in detail inAppendix C.

The impact absorption properties of a stretch-dominated structure suchas the hollow cell structure 1 are determined by the material used toform the structure, and the specific geometry of the structure: i.e.cell size, cell wall thickness, cell width and cell length as shown inFIG. 4.

If used in an impact-absorbing structure such as a helmet, the latticeis designed so that the axial part of the cell is always perpendicularto the surface of the head. This is important as the crush strength ofhoneycomb significantly diminishes as the impact angle increases awayfrom perpendicular to the axial part of the cell.

If the cell dimensions are known, a value of hollow cell relativedensity (or volume fraction solid) can be calculated using the equationshown below:

$\frac{p^{*}}{p_{s}} = {{\left\{ \frac{\frac{h}{l} + 2}{2*\left( {\frac{h}{l} + {\sin \; \theta}} \right)\cos \; \theta} \right\} \frac{t}{l}} = {{\frac{2}{\sqrt{3}}\frac{t}{l}\left( {1 - {\frac{1}{2\sqrt{3}}\frac{t}{l}}} \right)} = {\frac{2}{\sqrt{3}}\frac{t}{l}}}}$

For the particular embodiments of the honeycomb structure 1 of theinvention described, h is assigned a value of 1, θ has a value of 30(degrees), and the ratio of cell wall thickness (t) to cell length (l)is significantly small.

A proposed use for the hollow cell structure 1 would be in bicyclehelmets. The material used to create the hollow cell structure 1 in thisembodiment is Nylon 11 and ST Elastomer. This is a readily availablematerial, which is lightweight, easily formed and malleable, and istherefore suitable or at least analogous to the type of material thatwould be used for mass-manufactured helmets.

The hollow cell structure 1 was manufactured by additive manufacturing.The process is briefly described in Appendix B.

Tests were carried out as detailed in Appendix A, and Appendix D, withthe objective of determining how varying the relative density of thehoneycomb hollow cell structure 1 (this type of structure also known as‘out-of-plane honeycomb’) would affect the hollow cell structure 1 whensubjected to impact testing. As shown in Appendix A, the relativedensity was varied between 0.1 and 0.33 by changing the cell size (s)from between a minimum of 6 mm and a maximum of 20 mm, with the wallthickness maintained at a constant 1 mm.

The results indicate that an acceptable range of optimum relativedensities lies between 0.125 and 0.175 for this material and for theparticular cell/lattice size and shape used during testing, for thereasons outlined in the ‘Results from Impact Testing’ section ofAppendix A, and Appendix D. The results indicate that the cell size,cell wall thickness, cell width and cell length can be freely variedrelative to one another, and as long as the relative density liesbetween 0.03 and 0.17, then the structure will provide optimised impactabsorption properties.

As outlined in the ‘background’ section, helmet design is generally atrade-off between the overall weight of a helmet, and theimpact-absorbing properties. Based on the results of the tests detailedin Appendix A and Appendix D, a helmet such as helmet 5 shown in FIGS. 3and 4, constructed using a structure the same as or similar to the innerimpact resistant liner 7 (formed as a hexagonal hollow-cellstretch-dominated structure) covered by an outer shell 6, formed fromnylon 12 or a similar material, will provide a lightweight structurecapable of meeting and exceeding the relevant standards for impactabsorption, in particular BS EN 1078. The test results indicate theelastic-plastic honeycomb has a 3× greater EPV than a typical expandedpolystyrene helmet. This is clearly shown by the plots of theexperimental results shown in FIGS. 13 and 14.

The reasons can be summarised as follows:

-   -   Stretch dominated structures rely on ‘hard’ modes of deformation        through compression and tension. A long plateau force is        achieved, as the stress response softens as the area increases.    -   Stretch dominated structures have higher specific strength for        the same relative density, so it is possible to increase the        densification strain, as the relative density is lower in a        stretch dominated structure.    -   All stretch dominated structures have similar mechanical        responses. Therefore, an impact-absorbing structure can be        formed from any appropriate material and at any shape and size        (e.g. all honeycomb topology and materials), and will still        provide the advantages as outlined above.

As briefly noted above, where 2D hollow cells are referred to in thisspecification, this indicates a three-dimensional structure, with thecells of the structure formed in such a way as to have depth, but sothat when viewed at a certain angle the cells will have a uniform oridentical cross-section at any position perpendicular to the view angle.That is, a cross section taken at any position would be identical to onetaken at any other position. For example, a honeycomb cell structureviewed in plan or from directly above will provide a uniformcross-section at any depth through the cells. It should also be notedthat when a structure is referred to as ‘stretch-dominated’, this isaccording to the Maxwell criterion as outlined herein. It should furtherbe noted that the phrases ‘relative density’ and ‘volume fraction solid’essentially have the same meaning and are used interchangeably withinthis specification.

1. An impact absorbing structure, comprising a unitary material formedas a stretch-dominated hollow cell structure wherein at least aplurality of cells are configured to tessellate with a cell axis normalto the surface or out-of-plane and the unitary material has a relativedensity substantially between 0.05 and 0.15.
 2. An impact absorbingstructure as claimed in claim 1 wherein substantially all the cells ofthe hollow cell structure are 2D hollow-cells.
 3. An impact absorbingstructure as claimed in claim 2 wherein substantially all the cells arealigned substantially out of plane.
 4. An impact absorbing structure asclaimed in claim 1 wherein the cells are formed as a micro-trusslattice.
 5. An impact absorbing structure as claimed in claim 1 whereinthe cells are formed as a crystal lattice structure.
 6. (canceled) 7.(canceled)
 8. An impact absorbing structure as claimed in claim 1wherein at least a plurality of the cells are hexagonal.
 9. An impactabsorbing structure as claimed in claim 1 wherein at least a pluralityof the cells are triangular.
 10. An impact absorbing structure asclaimed in claim 1 wherein at least a plurality of the cells are square.11. An impact absorbing structure as claimed in claim 1 wherein at leasta plurality of the cells are a combination of octagons and squaresco-located in a tessellating pattern.
 12. (canceled)
 13. (canceled) 14.An impact absorbing structure as claimed in claim 1 wherein the ratio ofcell wall thickness to cell length is significantly small.
 15. An impactabsorbing structure as claimed in claim 14 wherein the wall has amaximum thickness of substantially 1 mm.
 16. An impact absorbingstructure as claimed in claim 1 wherein the unitary material is apolymer material.
 17. An impact absorbing structure as claimed in claim16 wherein the unitary material is an elastomer.
 18. An impact absorbingstructure as claimed in claim 16 wherein the unitary material iselastic-plastic and elastic-brittle.
 19. An impact absorbing structureas claimed in claim 16 wherein the unitary material is selected from thegroup consisting of Nylon 11 and ST Elastomer.
 20. (canceled) 21.(canceled)
 22. A helmet, comprising an inner impact resistant liner atleast partly formed from an impact absorbing structure as claimed inclaim
 1. 23. A helmet as claimed in claim 22 further comprising an outershell formed to substantially cover the inner impact resistant liner.24. A helmet as claimed in claim 22 further comprising an outer shellformed to substantially cover the inner impact resistant liner whereinthe outer shell is at least partly formed from a composite material. 25.A helmet as claimed in claim 22 further comprising an outer shell formedto substantially cover the inner impact resistant liner wherein theouter shell is at least partly formed from a thermoplastic material. 26.A helmet as claimed in claim 22 further comprising an outer shell formedto substantially cover the inner impact resistant liner wherein at leastone vent slot is formed in the outer shell. 27-50. (canceled)